A glow plug is known to assist starting of internal combustion engines such as diesel engines. The glow plug includes a housing, which is inserted and fixed into a through-hole formed in a cylinder of an engine; a heater, which protrudes from a tip of the housing into a combustion chamber (or auxiliary combustion chamber) of the cylinder; and a power supply shaft, which is provided in the housing and is a conductor that supplies power from an external power source to the heater. Before the starting of the engine and at the time of the starting of the engine, the heater receives power supplied from the power source in order to generate heat, thereby heating the inside of the combustion chamber and aiding the starting of the engine.
Another known glow plug has a housing in which a piezoresistive element, which receives combustion pressure in the combustion chamber from the heater, and a printed circuit board, on which a circuit that detects a resistance value of the piezoresistive element is provided, are provided (See Patent Document 1, for example).